Yes the internal disc spins 360 degrees in either direction. I haven't done any research on it yet. It was one of those things where I was looking for parts,saw it, and had to have it since the price was right.

I'm just guessing but I would think it would have been used by amateur pilots or navigators.

Edit: I saw one as a keychain and with the name of a supplier on the disc it makes sense that this was some type of sales give away or something like that.

Altimeters are aircraft cockpit mounted 'gages' that display altitude. Servo Altimeters replaced traditional altimeters because they're more accurate during changing aircraft condition such as take off or landing. They took information from various aircraft conditions and gave a more accurate altitude. Servo Altimeters have generally been replaced by modern flight computers.

The device pictured above is not a Servo Altimeter but appears to be an advertising gimmick/give away to promote Bulova Servo Altimeters. On one side a Bulova Servo Altimeter appears to be pictured. The other side is a pocket sized ground speed calculator that a pilot could use to estimate his/her ground speed.